Dielectric relaxations in PEEK by combined dynamic dielectric spectroscopy and thermally stimulated current

The molecular dynamics of a quenched poly (ether ether ketone) (PEEK) was studied over a broad frequency range from 10-3 to 106 Hz by combining dynamic dielectric spectroscopy (DDS) and thermo-stimulated current (TSC) analysis. The dielectric relaxation losses e00 KK has been determined from the real part e0 T(x) thanks to Kramers–Kronig transform. In this way, conduction and relaxation processes can be analyzed independently. Two secondary dipolar relaxations, the c and the b modes, corresponding to non-cooperative localized molecular mobility have been pointed out. The main a relaxation appeared close to the glass transition temperature as determined by DSC; it has been attributed to the delocalized cooperative mobility of the free amorphous phase. The relaxation times of dielectric relaxations determined with TSC at low frequency converge with relaxation times extracted from DDS at high frequency. This correlation emphasized continuity of mobility kinetics between vitreous and liquid state. The dielectric spectroscopy exhibits the ac relaxation, near 443 K, which has been associated with the rigid amorphous phase confined by crystallites. This present experiment demonstrates coherence of the dynamics of the PEEK heterogeneous amorphous phase between glassy and liquid state and significantly improve the knowledge of molecular/dynamic structure relationships

Preparation of single-molecule particles of water-soluble polymers

Single-molecule particles of a series of water-soluble polymers are prepared via a spray dryer from dilute polymer aqueous solutions. Two of these polymers, poly(acrylic acid) and polyacrylamide, are used as examples to illustrate the preparation and identification of single-chain molecules. Scanning electron microscopy shows that these dried particles possess diameters ranging from 1 to 4 mum. Transmission electron microscopy reveals that each particle consists of numerous distinctly separated small particles with a radius of a few nanometers. Each of these particles consists of a single-chain molecule, a conclusion based on calculations of volume involving the polymer molecular weight. This method lends itself to the preparation of single-molecule particles of water-soluble polymers in mass quantity, which may provide a gateway to studying the chemistry and physics of polymer single-molecules in a condensed state. (C) 2001 Elsevier Science Ltd. All rights reserved

Glass transition behavior of polystyrene blocks in the cores of collapsed dry micelles tethered by poly(dimethylsiloxane) coronae in a PS-b-PDMS diblock copolymer

The micellization of a polystyrene-b-poly(dimethylsiloxane) (PS-b-PDMS) diblock copolymer with the number-average molecular weights (M-n) of 193 000 g/mol for PS block and 39 000 g/mol for PDMS block and the glass transition behavior of the PS blocks in the cores of collapsed dry micelles with PDMS tethered coronae were studied. A range of micelle morphologies was observed by varying the micellization conditions and varying the ratio of the methylene chloride (MC) solvent and the n-dodecane (D) selective nonsolvent. MC is a solvent for both of the blocks, but D is a selective solvent for only the PDMS block. After completely removing the solvents below the glass transition temperature (T-g) of the initially solvated PS blocks in MC via extraction and vacuum-drying, the collapsed dry micelles, which were originally spheres and cylinders, became "tablets" or "ribbons". The diameters of the "tablets" and widths of the "ribbons" were approximately 50 and 100 nm, respectively. The thicknesses of these textures ranged from 20 to 40 nm. The PS block cores of the dried micelles exhibited a Tg very close to that of the homo-PS bulk samples with the same M-n. However, for a broad temperature range below the T-g(PS), the collapsed dry micelles possessed heat capacity (C-p) values higher than the addition scheme calculated Cp based on PS and PDMS in their solid and liquid bulk states; whereas the C-p values of the liquid state above the T-g(PS) fit well with the calculated data. These observations support the idea of PS interface layers with mobility induced by the liquid PDMS coronae such that only the inner cores of the PS blocks were in the glassy state. In a temperature region between 0 and 50 degreesC, the lower limit of the interface thickness (i.e., assuming that the mobile PS layers possessed the same mobility as the PS liquid) was estimated to be approximately 2 nm for the collapsed dry micelles. The interface thickness slightly increased with increasing temperature